1. Field of the Invention
This invention relates to coated cemented carbides and more particularly, it is concerned with coated cemented carbides having a hard coating film structure of increased wear resistance as well as high toughness, used as cutting tools.
2. Description of the Prior Art
The requirement of a higher wear resistance for cemented carbides used for cutting tools is increasing each year. Thus, it has hitherto been proposed to coat cemented carbides with single layers or multilayers of TiC, TiCN, TiN, Al.sub.2 O.sub.3, and the like by chemical vapor deposition methods (which will hereinafter be referred to as CVD method) as described in, for example, Japanese Patent Publication No. 24982/1976. It has also been proposed to coat cemented carbides with single layers or multilayers of TiC, TiCN, TiN and the like by physical vapor deposition methods (which will hereinafter be referred to as PVD method) as described in, for example, Japanese Patent Publication No. 18474/1984.
In these coated cemented carbides, however, the surface of the cemented carbides such as WC-TiC-Co, as a substrate, is coated with a material with a higher hardness but more brittleness than the cemented carbides and accordingly, there arises the problem that the toughness is lowered as a whole and when used as a cutting tool, the cutting edge tends to break with fluctuation of load stress. Furthermore, it is known that the toughness is lowered to a greater extent when the coated cemented carbide has a larger bonding strength between the substrate and coating film.
In the case of coated cemented carbides produced by the CVD method, in general, the lowering of the toughness is greater because the coating temperature is ordinarily higher, e.g. 1000.degree. C., resulting in reaction of the substrate and coating film or coarsening of the grain size of the film itself. On the contrary, in the case of coated cemented carbides produced by the PVD method, the lowering of the toughness is less than in the case of the CVD method because the coating temperature is lower, e.g. 200.degree. to 700.degree. C. than in the case of the CVD method, thus resulting in suppression of the reaction between the substrate and coating film and maintaining a finer grain size of the film itself. Therefore, the coated cemented carbides produced by the PVD method have been used for milling cutters with many repetitions, more than those by the CVD method.
A cemented carbide coated with only TiN, such as the coated cemented carbide produced by the PVD method, is inferior in flank wear because TiN itself has a lower hardness than TiC. A cemented carbide coated with only TiC is excellent in flank wear, but inferior in crater depth because of its lower chemical reaction resistance. Accordingly, many efforts to make the best use of only the strong points thereof have been made by coating TiCN having intermediate properties between TiC and TiN or by coating these materials in laminated layers or multilayer. In the case of coating such a multilayer, it is preferable to arrange a TiN layer at a position in direct contact with a workpiece to be cut, i.e. as an outermost layer so as to utilize its excellent chemical reaction resistance, and to position a TiC layer excellent in flank wear inside the TiN layer. In the case of providing a TiCN layer, it is preferable to position it between a TiC layer and a TiN layer as an outermost layer. The reason therefor is that TiCN, having intermediate properties between TiC and TiN, should be positioned inside the outermost layer in order to hold the effect of TiN as the outermost layer, and outside the TiC layer in order to make the best use of the TiN-like properties of TiCN. Therefore, film structures such as cemented carbide/TiC/TiN and cemented carbide/TiC/TiCN/TiN have hitherto been considered suitable in order to impart resistances to flank wear and crater depth.
However, the inventors have found that the above described film structure has a problem with respect to another important property for cutting tools, i.e. toughness. That is to say, the problem is that when a TiC layer is contacted with a cemented carbide alloy as a substrate, a diffusion takes place between WC contained in the substrate and TiC as the coating layer even in the PVD method wherein coating is possible at a lower temperature than in the CVD method, and the bonding strength is thus increased but the toughness, as a tool, is lowered to result in increased breakage, in particular, in use under conditions wherein load fluctuations occur frequently.